This invention relates to a method of detecting surface defects of a hot metal body by using radiation emitted by the hot metal body and external light projected on and reflected by the same body, and to an apparatus for performing the surface defect detection method.
Conventional optical or televisional inspection methods for the detection of surface defects of a hot metal body, such as hot-rolled steel sheet, include the so-called passive method in which the surface defects are detected by analyzing a video signal derived from radiation such as infrared radiation emitted by the hot metal and the so-called active method in which external light is projected on the hot metal surface to produce a video signal from the reflected light.
In general hot metal surface defects attributed to material abnormality such as scabbing, scaling or brick inclusion are relatively easy to detect by the aforementioned passive method, but the surface defects classified as geometric abnormality such as simple dents are rather difficult to accurately detect by the same method because the difference in temperature between the defective region and the normal region of the hot metal surface is so small that an expected defect-indicating signal cannot easily be discriminated from noise signals. In the case of the active method it is relatively easy to detect the surface defects of the geometric abnormality type, but it is difficult to accurately detect the surface defects attributed to material abnormality by this method because in this case signals indicative of such surface defects cannot easily be discriminated from noise signals.
Thus, the passive method and the active method each have merits and demerits, and either of these methods is not truly suitable for accurate detection of surface defects of every type because the rate of false detection becomes considerably high as to surface defects of a specific type. The obstructive noise signals are mostly attributed to metal oxide that is inevitably formed on the hot metal surface to be inspected.
Japanese patent application provisional publication No. 49-131192 (1973) proposes to jointly employ the passive method and the active method with a view to detecting the surface defects of one type by using self-emitted light and the surface defects of other type by using the reflected light. However, also in this case noise signals offer considerable obstruction to accurate detection of surface defects of every type, partly because the reflected light is always received together with the self-emitted light.
In the above described conventional surface defect detection methods it is a common practice to perform a descaling treatment prior to the inspection for surface defects to thereby reduce noise signals at the stage of producing a video signal. However, even though such a pretreatment is performed it is difficult to accomplish complete removal of scale from the hot metal surface, and accordingly the problem of high rate of false detection remains unsolved.